Use of branched polyacids in dental compounds

ABSTRACT

The invention relates to dental compounds that contain at least one polyacid or at least one polyacid and at least one salt of at least one polyacid. The polyacids used have at least one branch point in the polymer structure and are water-soluble and not cross-linked and their repeat units consist to at least 80 mole % of acrylic acid segments.

[0001] The invention relates to the use of branched, noncrosslinked,water-soluble polyacids in dental products, in particular in dentalcements, such as zinc polycarboxylate cements, glass ionomer cements(GICs), resin-modified glass ionomer cements (RM-GICs) and compomers,insofar as they can be formulated with aqueous polyacid solutions.

[0002] Polyacids have been used for a long time in the formulation ofcurable dental materials. For example, polyacrylic acid was used asground substance in zinc polycarboxylate cements (D. C. Smith, Br. Dent.J., 125 (1968), 381-384) or also in glass ionomer cements (ASPA I,Wilson and Kent, 1969; DE 20 615 13 A1).

[0003] While zinc polycarboxylate cements to this day are based onpolyacrylic acid, the glass ionomer cements were developed further withregard to the chemical composition of the polyacid.

[0004] For example, copolymeric acids of acrylic acid and itaconic acidfor use in glass ionomer cements are known from S. Crisp, B. E. Kent, B.G. Lewis, A. J. Ferner and A. D. Wilson, J. Dent, Res., 59, (1980),1055-1063. In addition, glass ionomer cements based on a copolymer ofacrylic acid and maleic acid are disclosed in EP 0 024 056 A1.

[0005] Highly concentrated, for example approx. 30 to 50%, aqueoussolutions of unbranched polyacids are usually used in dental materials,for example in polyelectrolyte cements and especially in zincpolycarboxylate cements and glass ionomer cements.

[0006] Highly branched, partially crosslinked polyacids for use indental products are disclosed in JP 4-221 303 and JP 4-221 304. However,these polyacids are no longer sufficiently soluble in water to becompletely available for the setting reaction in dental products, sothat unreacted defects arise which weaken the set material.

[0007] The overall proportion of polyacid in dental cements generallyhas to be chosen to be as high as possible in order to achieve themaximum strengths of the cured cement. At the same time, care must betaken that the system exhibits a manageable viscosity on mixing andapplying. This is important in particular in preparations which will beexpelled into mixing capsules, since the mixed material has to flowthrough an outlet nozzle.

[0008] The following approaches for optimizing are known in thisconnection:

[0009] 1. In GICs, a proportion of the polyacid used is mixed into thepowder. However, the proportion in the powder is limited, in particularin reinforced glass ionomer filling cements, since only a littlepolyacid from the powder can be dissolved on mixing with GIC liquid inthe short processing time necessary when used in a dental surgery.Undissolved amounts of polyacid from the powder act in the cement asdefects and consequently lead to a weakening of the material.

[0010] 2. A reduction in the overall content of polyacid in dentalcements is generally, and especially in reinforced GICs, accompanied byan obvious deterioration in the mechanical strength of the cured cement.

[0011] The polyacid most frequently employed in dental products isunbranched polyacrylic acid. However, in concentrated aqueous solution,this polyacid has a tendency to undergo irreversible gelling. Suchgelled polyacrylic acid solutions cannot be employed for use in dentalcements. Methanol has been added to the polyacrylic acid solutions tocounter this effect, but methanol, on the one hand, is poisonous and, onthe other hand, leads to discoloring of the cement in the oralenvironment.

[0012] 3. An additional approach to this solution consisted in usingcopolymers of acrylic acid and itaconic acid or maleic acid for dentalcement liquids. The gelling could thereby be prevented. However, thisresulted in some undesired properties for the dental cements preparedfrom them, such as deficient self-adhesion of the cements to the dentalhard substance and, in particular in reinforced glass ionomer cements infilling therapy, 10 to 20% lower bending strengths and because of thatreduced long-term performance.

[0013] That is why, for use in dental products, there exists a demandfor polyacids which are miscible with aqueous solutions, which show notendency to gel and which lead to excellent physical properties of thecured dental products.

[0014] Surprisingly, it has now been found that polyacids in which atleast 80 mol % of the repeat units are formed from acrylic acid segmentsand which simultaneously exhibit a branched but noncrosslinked polymerstructure do not gel, prevent the disadvantages known in the state ofthe art and are for this reason outstandingly suitable for use in dentalproducts.

[0015] The terms “including” and “comprising” introduce, in the contextof this application, an enumeration which is not comprehensive. The term“one” is equivalent to the statement of “at least one”.

[0016] The dental materials are normally curable materials, i.e.materials which in accordance with the requirements change in a periodof time of 30 sec to 30 min, preferably of 2 min to 10 min, from aviscous to a nonviscous condition. The curing reaction can, for example,be brought about by a cement reaction, a crosslinking reaction and/or apolymerization reaction.

[0017] A viscous condition, in contrast to a nonviscous condition, inthe above mearing then exists if the material can be processed orapplied with application devices familiar to dentists, such as aspatula, syringe or mixing capsule.

[0018] The term “gelling” should be understood, within the meaning ofthis invention, as meaning a process in which the aqueous solution of apolyacid changes from a flowable to a rigid form, this operation beingirreversible.

[0019] The term “polyacids” should be understood, within the context ofthis invention, as meaning polymers and copolymers which exhibit morethan three acid groups per polymer molecule and optionally otheradditional functional groups.

[0020] The term “acid groups” is to be understood as meaning inparticular carboxylic acid groups, phosphonic acid groups, phosphoricacid groups or sulfonic acid groups.

[0021] The term “branched polyacids” is to be understood as meaningpolyacids which comprise at least one branching point in the polymerstructure. Examples of branched polymer structures are star polymers,comb polymers, graft polymers, long-chain-branched polymers,short-chain-branched polymers, hyperbranched polymers or dendrimers.

[0022] A side group which sticks out from a polymer chain is not abranching point according to the invention. Crosslinkages produced bycomplexing reactions are also not branchings within the meaning of theinvention. Repeat units are always connected from a branching point.According to the invention, the term “branching point” is to beunderstood as meaning a position in the polymer backbone from which,covalently bonded, at least two, preferably three, polymer residuesstart out, these comprising at least three repeat units.

[0023] A polyacid is then water-soluble within the meaning of theinvention if an aqueous, completely clear, solution comprising at leastone percent by weight of a polyacid can be prepared. In contrastthereto, crosslinked polymers are, as is known, generally insolubleunder standard conditions in the solvents current in the dental field.

[0024] Star polymers or comb polymers are preferred for the invention,particularly preferably star polymers.

[0025] In star polymers, at least three in themselves unbranched polymerchains, preferably with at least three repeat units, are combinedtogether via a common branching point. A branching point can be apolyvalent atom or can have a low-molecular-weight molecular structure,such as a benzene ring or a cyclohexane ring.

[0026] In comb polymers, at least two polymer chains are connectedlaterally to a common main polymer chain which in itself is unbranched.

[0027] Dental products according to the invention are accordingly thoseincluding at least one polyacid, optionally together with at least onesalt of at least one polyacid, in which the polyacids used have at leastone, preferably one, branching point in the polymer structure, arewater-soluble and noncrosslinked and, with regard to the repeat units,comprise at least 80 mol %, preferably at least 90 mol %, particularlypreferably at least 95 mol %, and very particularly preferably 100 mol %acrylic acid segments.

[0028] The term “repeat units” is to be understood as meaning not thestructural units of the branching points but exclusively the segmentswhich repeat themselves of the preferably linear polymer chains startingout from the branching points. In copolymeric acids, the differentrepeat units are usually arranged randomly.

[0029] The molecular weight distribution Mw/Mn of the polyacids is ineach case, according to the method of preparation, usually between 1.0and 10.0, preferably in the range from 1.5 to 6.0; however, highervalues can also be accepted.

[0030] The term “Mw” is to be understood, within the meaning of thisapplication, as meaning the weight-average molecular weight determinedby means of aqueous gel permeation chromatography (GPC), for whichapplies:$M_{W} = \frac{\sum\limits_{i}^{\quad}\quad {n_{i}M_{i}^{2}}}{\sum\limits_{\quad i}^{\quad}\quad {n_{i}M_{i}}}$

[0031] in which n_(i)=number of the polymer chains and M_(i)=molar massof the polymer chain.

[0032] The term “Mn” is to be understood, within the meaning of thisapplication, as meaning the number-average molecular weight determinedby means of aqueous GPC, for which applies:${Mn} = {\frac{\sum\limits_{i}^{\quad}\quad {n_{i}M_{i}}}{\sum\limits_{\quad i}^{\quad}\quad n_{i}} = \frac{\sum\limits_{i}^{\quad}\quad {n_{i}M_{i}}}{n}}$

[0033] in which n=total number of the polymer chains in a sample.

[0034] An additional advantage of the use according to the invention ofbranched polyacids is the high bending strengths which can be obtained.It is possible, with glass ionomer filling cements, for example, toobtain bending strengths which are shifted to the high level of theknown systems based on linear polyacrylic acid.

[0035] Curable dental materials or cements can be prepared with thepolyacids described, which dental materials or cements, for example,exhibit via a compressive strength ranging from 200 to 260 MPa (measuredaccording to ISO 9971), a bending strength ranging from 30 to 50 MPa(measured analogously to ISO 4049 with test specimens 12 mm long) and/ora surface hardness ranging from 350 to 510 MPa (measured according toDIN 53456).

[0036] A further advantage of the dental products according to theinvention comprising branched polyacids is that distinctly reducedviscosities are found in a concentrated aqueous solution of the branchedpolyacids, which leads to a simplified miscibility of powder and liquid.

[0037] In addition, the dental products according to the invention havethe advantage that no toxic or other undesired constituents or additivesare present.

[0038] The branched polyacids present in the dental products accordingto the invention are accessible in principle according to differentmethods of preparation.

[0039] “Arm-first” and “core-first” synthetic principles known for starpolymers (K. Matyjaszewski et al., Macromolecules, 1999, 32, p. 6526)are valid in principle for comb polymers as well.

[0040] 1. Star and comb polymeric acids can be prepared according to the“arm-first method” from linear polyacids or their derivatives which canbe converted to polyacids, which have a functional chain end, byreactive coupling to a polyfunctional low-molecular-weight, for examplecyclic, compound or oligomeric, for example macrocyclic or linearpolymeric, compound. Mention may be made, as general examples, of:

[0041] a) preparation of polyacrylic acid with an amino end group, forexample by free radical polymerization in the presence of anamino-functional transfer reagent (A. Akar, N. Öz, Angew. Makromol,Chem., 273, 1999, 12-14) and subsequent reaction, for example withpoly-functional carbonyl chlorides or carboxylic anhydrides, such asbenzenetricarbonyl trichloride, butanetetracarbonyl tetrachloride,oligomeric maleic anhydride, acryloyl chloride polymers, such aspoly(acrylic acid-co-acryloyl chloride), polyfunctional benzyl halides,such as 1,2,4,5-tetrakis(bromomethyl)benzene,hexakis(bromomethyl)benzene, with tetra(bromomethyl)pentaerythritol orother low-molecular-weight oligomeric or polymeric coupling reagents.1,4-Dioxane is frequently particularly suitable for carrying out thecoupling reaction since it constitutes a good solvent for polyacrylicacid.

[0042] b) Polyacrylic acid derivatives, such as poly(tert-butylacrylate), with end group functionalizing can also be used as functionalpolyacid derivatives, in which derivatives alcohol or thiol groups are,for example, also possible as end groups, in addition to the preferredprimary or secondary amino groups. After a coupling of these polymersanalogous to 1.a), the tert-butyl ester groups have to be selectivelycleaved. This can occur, for example, by heating the polymer in adioxane solution with hydrochloric acid (K. A. Davis et al., PolymerPrepr., 2/1999, 430).

[0043] c) Poly(butyl acrylate) prepared by anionic polymerization (P.Banerjee and A. M. Mayes, Macromolecules, 1998, 31, 7966) can also beused as polyacrylic acid derivative and is reacted as anionic entityaccording to 1.a) with a coupling reagent. The butyl ester groups aresubsequently removed according to conventional methods, for example byacid or alkaline hydrolysis in an aqueous/organic medium, such as adioxane/water mixture.

[0044] 2. Star and comb polymeric acids can be prepared according to the“core-first method” by grafting acid monomers or their derivatives whichcan be converted to acid segments to polyfunctionallow-molecular-weight, cyclic, oligomeric, macrocyclic or linearpolymeric initiators according to a living polymerization mechanism,preferably an anionic or living free-radical mechanism. The livingpolymerization mechanism in this connection avoids the formation of morehighly branched or crosslinked polymers. Atom Transfer Polymerization(ATRP) can preferably be applied in living free-radical polymerization.

[0045] a) Polyfunctional initiators for the preparation of acrylate starpolymers are described, for example, in C. Pugh et al., Polymer Prepr.,2/1999, 108; D. M. Haddleton et al., New J. Chem., 1999, 23, 477; Heiseet al., Polymer Prepr., 1/1999, 452; A. Heise et al., Macromolecules,1999, 32, 231; S. Angot et al., Macromolecules, 1998, 31, 7218. Thecleavage of the polyacrylate ester groups subsequently necessary can becarried out according to the usual methods.

[0046] b) Polyfunctional initiators for the synthesis of acrylate combpolymers can, for example, be copolymers or oligomers of acrylates and4-chloromethylstyrene, the benzylic chloride groups being suitable asATRP initiator for acrylates. After the ATRP, the acrylate groups in themain chain and in the side chains can be cleaved according to the usualmethods to give acrylic acid segments.

[0047] In addition to the acrylic acid segments, additional acid repeatunits or also other functional or nonfunctional repeat units and groupscan be present in the polyacids in a total of 0.1 to 20 mol %,preferably 0.1 to 10 mol %, particularly preferably 0.1 to 5 mol %.

[0048] The additional acid repeat units can, for example, be producedfrom the copolymerization of acrylic acid with methacrylic acid, maleicacid, maleic anhydride, fumaric acid, itaconic acid, itaconic anhydride,glutaconic acid, citridic acid, citraconic acid, methaconic acid, tiglicacid, crotonic acid, muconic acid, isocrotonic acid, 3-butenoic acid,cinnamic acid, styrenecarboxylic acid, vinylphthalic acid, abietinicacid, styrenesulfonic acid, styrenephosphonic acid,1-phenylvinylphosphonic acid, vinylphosphonic acid,vinylidenediphosphonic acid, vinylsulfonic acid, vinyl phosphate orother monomers with acid functional groups and substituted derivativesthereof, especially esters or amides or imides of the acids mentionedwith up to 10 carbon atoms in the alkoxide residue or amine residue orimine residue, for example maleimide, N-methylmaleimide,N-ethylmaleimide, acrylamide, N-methylacrylamide,N,N-dimethylacrylamide, N-ethylacrylamide, methyl acrylate, ethylacrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate,isobutyl acrylate or tert-butyl acrylate.

[0049] Examples of other functional repeat units are structural segmentsproduced from vinyl acetate, vinyl ethers, for example vinyl methylether, vinyl alcohol, butadiene, isoprene, styrene, vinyl chloride,vinylidene dichloride, vinyl fluoride or vinylidene difluoride.

[0050] Examples of nonfunctional repeat units are structural unitsproduced from ethylene, propylene, tetramethylene, 1-butene, 1-penteneor 1-hexene.

[0051] The polyacids can, according to the field of application, be usedas liquid or, if appropriate, additionally as solid, for exampleobtained through freeze drying or spray drying.

[0052] The polyacids usually exhibit a molecular weight Mw ranging from1000 to 1 000 000, preferably ranging from 10 000 to 500 000.

[0053] The polyacids are preferably used as aqueous solutions with 1 to70% by weight, preferably 20 to 60% by weight, particularly preferably30 to 50% by weight, at a temperature of 22° C.

[0054] The cations of the salts of the polyacids which can be usedaccording to the invention are taken from the group: alkali metalelements, alkaline earth metal elements, zinc, aluminum, scandium,yttrium and lanthanum. Na, Ca and Al salts are preferred in thisconnection.

[0055] Preferred dental materials in the context of this invention areeither formulated with a single component, such as compomers, or withtwo or more components, such as carboxylate cements, glass ionomercements and resin-modified glass ionomer cements, in which the liquidcomponents are stored separately from the solid components and are mixedimmediately before application.

[0056] At the same polyacid concentration, as a result of a reducedsolution viscosity of the polyacids according to the invention, higherP/L ratios can be set and still well managed (up to approximately 30%higher) than in the polyacids known in the state of the art. Thisregularly also results in improved material strengths (up toapproximately 20% improved values in compressive strength and bendingstrength).

[0057] Powder/liquid ratios which can be used range from 0.7 to 6,preferably from 1 to 5, particularly preferably from 1.5 to 3.5.

[0058] Particular preference is given to cements which, in the case ofglass ionomer cements, for example, can include the followingconstituents:

[0059] (A) 1 to 60% by weight, preferably 5 to 40% by weight,particularly preferably 10 to 30% by weight, of polyacids, optionallytogether with at least one salt of at least one polyacid, in which thepolyacids used have at least one, preferably one, branching point in thepolymer structure, are water-soluble and noncrosslinked and, with regardto the repeat units, comprise at least 80 mol %, preferably 90 mol %,particularly preferably 95 mol % and very particularly preferably 100mol % acrylic acid segments;

[0060] (B) 35 to 80% by weight, preferably 50 to 70% by weight, offillers;

[0061] (C) 0 to 20% by weight, preferably 1 to 10% by weight, ofadditives and auxiliaries;

[0062] (D) 5 to 40% by weight, preferably 9 to 30% by weight, of water.

[0063] The term “fillers” of the component (B) is to be understood asmainly reactive or nonreactive solids.

[0064] Suitable examples are reactive fluoroaluminosilicate glasses fromDE 20 61 513 A, DE 20 65 824 A, or reactive glasses which, on thesurface, in comparison with the average composition, are depleted incalcium ions, as described in DE 29 29 121 A.

[0065] The last-named glasses are especially preferred and can exhibitthe following composition: Constituent Calculated as % by weight Si SiO₂20 to 60 Al Al₂O₃ 10 to 50 Ca CaO  1 to 40 F F  1 to 40 Na Na₂O  0 to 10P P₂O₅  0 to 10

[0066] and in addition a total of 0 to 20% by weight, calculated asoxides, of B, Bi, Zn, Mg, Sn, Ti, Zr, La or other trivalent lanthanides,K, W, Ge, and other additives, which do not impair the properties andare physiologically completely harmless.

[0067] In addition to the reactive glasses described above, inertfillers, such as quartz, can be used.

[0068] The term “component (C)” is to be understood as, for example,additives for accelerating and improving the curing, as are known fromDE 2 319 715 A. Preferably, chelating agents in the form of lowmolecular weight acid molecules, such as tartaric acid, are added.

[0069] Coloring pigments and other auxiliaries usual in the field ofglass ionomer cements, for example for improving the miscibility, arealso to be understood under “component (C)”.

[0070] Apart from their use in conventional glass ionomer cements, thepolyacids described here are also suitable for use in carboxylatecements.

[0071] In this connection, the compositions include, for example, thefollowing constituents:

[0072] (A) 1 to 60% by weight, preferably 5 to 40% by weight, ofpolyacids, particularly preferably 10 to 30% by weight, optionallytogether with at least one salt of at least one polyacid, in which thepolyacids used have at least one, preferably one, branching point in thepolymer structure, are water-soluble and noncrosslinked and, with regardto the repeat units, comprise at least 80 mol %, preferably 90 mol %,particularly preferably 95 mol % and very particularly preferably 100mol % acrylic acid segments;

[0073] (C) 0 to 20% by weight, preferably 1 to 10% by weight, ofadditives and auxiliaries;

[0074] (D) 10 to 40% by weight, preferably 15 to 30% by weight, ofwater;

[0075] (E) 30 to 80% by weight, preferably 44 to 70% by weight, of zincoxide.

[0076] Compomers or resin-modified glass ionomer cements comprising thepolyacids described here include, for example, the following components:

[0077] (A) 1 to 76% by weight, preferably 2 to 69.9% by weight,particularly preferably 10 to 40% by weight, of polyacids, optionallytogether with at least one salt of at least one polyacid, in which thepolyacids used have at least one, preferably one, branching point in thepolymer structure, are water-soluble and noncrosslinked and, with regardto the repeat units, comprise at least 80 mol %, preferably 90 mol %,particularly preferably 95 mol % and very particularly preferably 100mol % acrylic acid segments;

[0078] (D) 5 to 40% by weight, preferably 5 to 35% by weight, of water;

[0079] (F) 8.9 to 70% by weight, preferably 10 to 60% by weight, of oneor more radically polymerizable monomers;

[0080] (G) 10 to 90% by weight, preferably 15 to 87.9% by weight, offillers;

[0081] (H) 0.1 to 5% by weight, preferably 0.5 to 3% by weight, ofinitiators and optionally activators;

[0082] (I) 0 to 30% by weight, preferably 0.1 to 20% by weight, ofadditives, optionally pigments, thixotropic auxiliaries, plasticizers.

[0083] Mono-, di- or polyfunctional ethylenically unsaturated compounds,preferably based on acrylate and/or methacrylate, are used as component(F). These can comprise both monomeric and polymolecular oligomeric orpolymeric acrylates. In addition, they can be used in the formulationsalone or as mixtures.

[0084] Suitable monomers are, for example, the acrylic and methacrylicesters of mono-, di- or polyfunctional alcohols. The following arementioned as examples: 2-hydroxyethyl (meth)acrylate, methyl(meth)acrylate, isobutyl (meth)acrylate, ethylene glycoldi(meth)acrylate, triethylene glycol di(meth)acrylate (TEGDMA),hexanediol di(meth)acrylate, dodecanediol di(meth)acrylate andtrimethylolpropane tri(meth)acrylate.

[0085] Others which can advantageously be used are bisphenol Adi(meth)acrylate and the ethoxylated or propoxylated di(meth)acrylatesderived therefrom. In addition, the monomers described in U.S. Pat. No.3,066,112, based on bisphenol A (meth)acrylate and glycidyl(meth)acrylate or their derivatives arising from addition ofisocyanates, are suitable.

[0086] The diacrylic and dimethacrylic esters ofbis(hydroxymethyl)tricyclo[5.2.1.0^(2,6)]decane mentioned in DE 28 16823 C and the diacrylic and dimethacrylic esters of the compounds ofbis(hydroxymethyl)tricyclo[5.2.1.0^(2,6)]decane extended with 1 to 3ethylene oxide and/or propylene oxide units are also especiallysuitable.

[0087] Urethane (meth)acrylates, such as7,7,9-trimethyl-4,13-dioxo-5,12-diazahexadecane-1,16-dioxydimethacrylate (UDMA), can also be a constituent of this component.

[0088] Fillers according to component (G) can be inorganic fillers, forexample quartz, glass powder, water-insoluble fluorides, such as CaF₂,silica gels and silicic acid, especially pyrogenic silica, and theirgranulates. Cristobalite, calcium silicate, zirconium silicate,zeolites, including molecular sieves, metal oxide powders, such asaluminum or zinc oxides or their mixed oxides, barium sulfate, yttriumfluoride or calcium carbonate can also be used as fillers.

[0089] Fluoride-releasing fillers, for example complex inorganicfluorides of the general formula A_(n)MF_(m), as described in DE 44 45266 A, can also be used or added. A represents a mono- or polyvalentcation, M represents a metal from the main group or subgroup III, IV orV, n represents an integer from 1 to 3 and m represents an integer from4 to 6.

[0090] Organic fillers can also be a constituent of this component.

[0091] Those mentioned by way of example are conventional pearl-shapedpolymers and copolymers based on methyl methacrylate, which, forexample, are available from Rohm under the name “Plexidon” or “Plex”.

[0092] For improved incorporation in the polymer matrix, it can beadvantageous to render hydrophobic, using a silane, the fillersmentioned and optionally additives opaque to X-rays. The amount of thesilane used usually amounts to 0.5 to 10% by weight, with reference toinorganic fillers, preferably 1 to 6% by weight, very particularlypreferably 2 to 5% by weight, with reference to inorganic fillers.Normal hydrophobing agents are silanes, for exampletrimethoxymethacryloyl-oxypropylsilane. The maximum mean particle sizeof the inorganic fillers preferably amounts to 15 μm, in particular 8μm. Fillers with a mean particle size of <3 μm are very particularlypreferably used.

[0093] The term “initiators” according to component (H) is to beunderstood as initiator systems which bring about the radicalpolymerization of monomers, for example photoinitiators and/or what areknown as redox initiator systems and/or thermal initiators.

[0094] Examples of suitable photoinitiators are α-diketones, such ascamphorquinone, in conjunction with secondary and tertiary amines, ormono- and bisacylphosphine oxides, such as2,4,6-trimethylbenzoyldiphenylphosphine oxide andbis(2,6-dichlorobenzoyl)(4-n-propylphenyl)phosphine oxide. However,other compounds of this type, as are described in EP 0 073 413 A, EP 0007 508 A, EP 0 047 902 A, EP 0 057 474 A and EP 0 184 095 A, are alsosuitable.

[0095] Organic peroxide compounds together with “activators” aresuitable as redox initiator systems. Compounds such as lauroyl peroxide,benzoyl peroxide, p-chlorobenzoyl peroxide and p-methylbenzoyl peroxideare suitable in particular as organic peroxide compounds.

[0096] Tertiary aromatic amines, such as theN,N-bis(hydroxyalkyl)-3,5-xylidines known from U.S. Pat. No. 3,541,068,and the N,N-bis(hydroxyalkyl)-3,5-di(tert-butyl)anilines known from DE26 58 530 A, especiallyN,N-bis(β-hydroxybutyl)-3,5-di(tert-butyl)aniline, andN,N-bis(hydroxyalkyl)-3,4,5-trimethylanilines, for example, are suitableas activators.

[0097] Highly suitable activators are also the barbituric acids andbarbituric acid derivatives described in DE 14 95 520 C and the malonylsulfamides described in EP 0 059 451 A. Preferred malonyl sulfamides are2,6-dimethyl-4-isobutylmalonyl sulfamide, 2,6-diisobutyl-4-propylmalonylsulfamide, 2,6-dibutyl-4-propylmalonyl sulfamide,2,6-dimethyl-4-ethylmalonyl sulfamide and 2,6-dioctyl-4-isobutylmalonylsulfamide.

[0098] For further acceleration, the polymerization is in thisconnection preferably carried out in the presence of heavy metalcompounds and ionogenic halogen or pseudo-halogen. Copper is especiallysuitable as heavy metal and the chloride ion is especially suitable ashalide. The heavy metal is suitably used in the form of soluble organiccompounds. Likewise, the halide and pseudohalide ions are suitably usedin the form of soluble salts. Examples which may be mentioned are thesoluble amine hydrochlorides and quaternary ammonium chloride compounds.

[0099] If the dental materials according to the invention comprise aredox initiator system formed from organic peroxide and activator,peroxide and activator are then preferably present in parts of thedental material according to the invention which are spatially separatefrom one another, and they are homogeneously mixed with one another onlyimmediately before use. If organic peroxide, copper compound, halide andmalonyl sulfamide and/or barbituric acid are present side by side, thenit is particularly sensible for the organic peroxide, malonyl sulfamideand/or barbituric acid and the copper compound/halide combination to bepresent in three constituents spatially separate from one another. Forexample, the copper compound/halide combination, polymerizable monomersand fillers can be kneaded to a paste and the other components can bekneaded to two separate pastes in the above-described way in each casewith a small amount of fillers or in particular thixotropic auxiliaries,such as silanized silicic acid, and a plasticizer, for examplephthalate. On the other hand, the polymerizable monomers can also bepresent together with organic peroxide and fillers. Alternatively,organic peroxide, copper compound, halide and malonyl sulfamide and/orbarbituric acid can also be split up according to DE 199 28 238 A.

[0100] Soluble organic polymers can be used as representatives ofcomponent (I), for example for increasing the flexibility of thematerials. Poly(vinyl acetate) and the copolymers based on vinylchloride/vinyl acetate, vinyl chloride/vinyl isobutyl ether and vinylacetate/maleic acid dibutyl ether, for example, are suitable. Dibutyl,dioctyl and dinonyl phthalates or adipates and polymolecularpolyphthalic and adipic esters, for example, are highly suitable asadditional plasticizers. Modified layered silicates (bentonites) ororganic modifying agents, for example based on hydrogenated castor oils,can also be used, in addition to pyrogenic silicic acids, as thixotropicauxiliaries. Furthermore, inhibitors, as are described in EP 0 374 824 Aas component (d), can be included in the formulations as additives.

[0101] Likewise inventive are containers comprising dental productsincluding at least one polyacid, optionally together with at least onesalt of at least one polyacid, in which the polyacid used has at leastone branching point in the polymer structure, is water-soluble andnoncrosslinked and, with regard to the repeat units, consists of atleast 80 mol % acrylic acid segments. Such containers can, for example,be capsules, tubes, including screw-cap tubes, application syringes,application tips, cannulas, sachets, blister packs, cartons or glasscontainers. All containers in which several components can be storedseparately from one another are suitable in principle, whereby the term“separate storage” should be understood as also meaning the physicalseparation of the constituents via the different states of matter.

[0102] The invention is subsequently illustrated by examples, without itbeing limited in any way thereby.

[0103] The determination of the molar mass distribution was carried outin this connection via aqueous gel permeation chromatrography (GPC) at23° C. and pH=7 against polyacrylic acid sodium salt standard with theRID6a refractive index detector (Shimadzu) and comparative measurementwith the MiniDawn light scattering detector (Wyatt), the lastmeasurement as absolute measuring method making it possible todistinguish between unbranched and branched polyacids. Polyacrylic acidsodium salt standards (PSS) were used for calibrating the refractiveindex detector. The measured values were converted to free polyacrylicacid using the factor 0.766.

[0104] The samples were measured here as 0.05% aqueous solutions in thesolvent, a 0.9% by weight aqueous sodium nitrate solution, to which 200ppm of sodium azide are also added. The solutions were adjusted to pH=7by addition of sodium hydroxide.

[0105] A column combination of Hema3000, HemaBio1000 and HemaBio40 (PSS)was used to measure maximum molecular weights of up to 670 000 g/mol. Acolumn combination of Suprema1000, Hema3000 and HemaBio1000 (PSS) wasused with maximum molecular weights of up to 1 100 000 g/mol.

PREPARATIVE EXAMPLE 1 4-Arm Star Polyacrylic Acid (PAA-4)

[0106] 5.0 g (11.1 mmol) of 1,2,4,5-tetrakis(bromomethyl)benzene, 18.2 g(211 mmol) of methyl acrylate, 0.27 g of copper powder, 1.56 g ofcopper(II) triflate and 1.46 g ofN,N,N′,N″,N″-pentamethyldiethylenetriamine are mixed together under anitrogen atmosphere. The dark mixture is heated at 80° C. with stirringfor 17 hours. A ¹H NMR spectrum afterwards shows complete reaction ofthe benzyl bromide groups (disappearance of the signal at 4.6 ppm), sothat equal growth of the four arms can be assumed.

[0107] 200 g of methyl acrylate are added and the mixture is heated at80° C. for a further 20 hours. In the course of this, the reactionmixture becomes viscous. A monomer conversion of approximately 98% isestablished by means of a ¹H NMR spectrum.

[0108] The polymerizate is dissolved in 2 liters of methylene chlorideand successively extracted with 2M hydrochloric acid and water. Thefinal aqueous wash has a pH of 5.

[0109] Saponification is carried out by treating the organic solutionwith one part by volume of 4M aqueous potassium hydroxide solution andheating at 40° C. with vigorous stirring. The conversion is monitored by¹H NMR spectroscopy and is complete after 48 hours. The aqueous phase isseparated and treated several times with an acidic ion-exchangematerial.

[0110] The concentration is adjusted to 40% by weight by concentratingunder vacuum and monitoring by means of titration with lithiumhydroxide. The potassium content is less than 100 ppm by atomicabsorption spectroscopy.

[0111] Yield: 424 g (corresponds to a polymer yield of 92%).

[0112] The following values were established using a refractive indexdetector against a linear polyacrylic acid sodium salt standard (PAS) orwith a light scattering detector (LS):

[0113] Mw=133 000 (LS) or 88 000 (PAS)

[0114] Mn=78 000 (LS) or 44 000 (PAS)

[0115] Mw/Mn=1.7 (LS) or 2.0 (PAS).

PREPARATIVE EXAMPLE 2 6-Arm Star Polyacrylic Acid (PAA-6)

[0116] 1.0 g of hexakis(bromomethyl)benzene with 180 mg of copperpowder, 1 020 mg of copper(II) triflate, 960 mg ofN,N,N′,N″,N″-pentamethyldiethylenetriamine and 93.0 g of methyl acrylateare weighed out successively into a flask under a nitrogen atmosphere.The mixture is heated at 80° C. with stirring. After a reaction time offive days, the dark contents of the flask are very viscous. A conversionof >90% is established using ¹H NMR spectroscopy.

[0117] The batch is dissolved in 2 liters of methylene chloride andsuccessively extracted with 2M hydrochloric acid and water, so that thefinal aqueous wash has a pH of 5.

[0118] The organic phase is treated with one part by volume of 4Mpotassium hydroxide solution and heated at 40° C. for 3 days withvigorous stirring. The conversion of the saponification is afterwardscomplete according to ¹H NMR. The aqueous phase is treated with anacidic ion-exchange material and brought to a concentration of 32% byweight by concentrating under vacuum (titration with LiOH). Thepotassium content is less than 100 ppm by atomic absorptionspectroscopy.

[0119] Yield: 197 g (corresponds to a polymer yield of 81%).

[0120] The following values were established using a refractive indexdetector against a linear polyacrylic acid sodium salt standard (PAS) orwith a light scattering detector (LS):

[0121] Mw=460 000 (LS) or 117 000 (PAS)

[0122] Mn=182 000 (LS) or 47 000 (PAS)

[0123] Mw/Mn=2.5 (LS) or 2.5 (PAS).

[0124] The GPC results confirm the star structure.

[0125] Application Examples

[0126] The polyacids according to the preparation examples were testedin the application for dental glass ionomer cements.

[0127] Bending strengths were determined using the ISO 4049 standard, inwhich, however, test specimens with a length of 12 mm were used. In eachcase, mean values from series of five test specimens with a relativeerror of approximately ±10% per measured value were determined. Theprocedure used to determine the compressive strengths of the set cementswas that of the ISO 9917 standard. Surface hardnesses were determinedanalogously according to DIN 53456, the test specimens being stored inwater at 36° C. up to the time of measurement.

[0128] The viscosities of the aqueous polyacid solutions were measuredat 23° C. with a PK 100 viscometer from Haake.

[0129] Concentrations were determined by titration with lithiumhydroxide in the presence of 1% by weight of lithium chloride.

[0130] The storage stability was established by storage of the polyacidsolutions in glass flasks at ambient temperature (25° C.) and visualassessment of the samples at regular intervals of time. Several lots ofeach polyacid were stored, the analytical values of the various lots ofeach polyacid differing only slightly (deviations, maximum <10%).

[0131] The stated polyacids were spatulated with a standard glassionomer powder based on a calcium strontium fluoroaluminosilicate glass(glass ionomer cement glass) (Diamond Carve Powder, Kemdent, England)with a powder/liquid ratio of 3.5/1 (parts by weight).

[0132] Two commercial (Aldrich) unbranched polyacrylic acids (PAA-1a andPAA-1b), with different average molar masses, were used as comparativeexample.

[0133] The results are summarized in table 1: TABLE 1 PAA-4 PAA-6(according to (according to Polyacid PAA-1a PAA-1b the invention) theinvention) Concentration 42 35 44 40 [%] (±1) Mw 60000 250000 133000460000 (GPC-LS) Mn 12000 50000 78000 182000 (GPC-LS) Viscosity 8.0 8.28.4 7.7 [Pa · s] Bending 42 45 47 44 strength [MPa] Storage <6 months <6months >24 months >24 months stability as (3 of 5 (2 of 3 (all lots)(all lots) regards lots) lots) gelling

[0134] In spite of clearly higher average molar masses and higherconcentrations of the star polyacids, the bending strengths and theviscosities lie at the same level as in the reference substances(Comparison: PAA-4 with PAA-1a and PAA-6 with PAA-1b). However, the staracids also show a reliable storage stability over 24 months, while, ofthe unbranched reference acids, several lots were already irreversiblygelled within six months and were accordingly unusable.

1-11. (cancelled)
 12. A dental material comprising at least one polyacidor at least one polyacid together with at least one salt of at least onepolyacid, the polyacids employed having at least one branching point inthe polymer structure, being water-soluble and noncrosslinked and, withregard to the repeat units, comprising at least 80 mol % acrylic acidsegments.
 13. The dental material as claimed in claim 12, wherein thepolyacids comprise star polymers or comb polymers.
 14. The dentalmaterial of claim 12, wherein the polyacids are copolymers in which theproportion of the comonomers amounts to 0.1 to 20 mol %.
 15. A dentalmaterial including: A. 1 to 60% by weight of polyacids, in which thepolyacids used have at least one branching point in the polymerstructure, are water-soluble and noncrosslinked and, with regard to therepeat units, comprise at least 80 mol % acrylic acid segments; B. 35 to80% by weight of fillers; C. 0 to 20% by weight of additives andauxiliaries; D. 5 to 40% by weight of water.
 16. The dental material ofclaim 15, wherein polyacids, together with at least one salt of at leastone polyacid, being used as component (A), in which the polyacids usedhave at least one branching point in the polymer structure, arewater-soluble and noncrosslinked and, with regard to the repeat units,comprise at least 80 mol % acrylic acid segments.
 17. A dental materialincluding: A. 1 to 60% by weight of polyacids, in which the polyacidsused have at least one branching point in the polymer structure, arewater-soluble and noncrosslinked and, with regard to the repeat units,comprise at least 80 mol % acrylic acid segments; C. 0 to 20% by weightof additives and auxiliaries; D. 10 to 40% by weight of water; E. 30 to80% by weight of zinc oxide.
 18. The dental material as claimed in claim17, wherein polyacids, together with at least one salt of at least onepolyacid, being used as component (A), in which the polyacids used haveat least one branching point in the polymer structure, are water-solubleand noncrosslinked and, with regard to the repeat units, comprise atleast 80 mol % acrylic acid segments.
 19. A dental material including:A. 1 to 76% by weight of polyacids, in which the polyacids used have atleast one branching point in the polymer structure, are water-solubleand noncrosslinked and, with regard to the repeat units, comprise atleast 80 mol % acrylic acid segments; D. 5 to 40% by weight of water; F.8.9 to 70% by weight of one or more radically polymerizable monomers; G.10 to 90% by weight of fillers; H. 0.1 to 5% by weight of initiators andoptionally activators; I. 0 to 30% by weight of additives, optionallypigments, thixotropic auxiliaries, plasticizers.
 20. The dental materialas claimed in claim 19, wherein polyacids, together with at least onesalt of at least one polyacid, being used as component (A), in which thepolyacids used have at least one branching point in the polymerstructure, are water-soluble and noncrosslinked and, with regard to therepeat units, comprise at least 80 mol % acrylic acid segments.
 21. Acontainer including at least one component of a dental material asclaimed in claim
 12. 22. A method of preparing a dental materialcomprising using a water-soluble and noncrosslinked polyacid with atleast one branching point in the polymer structure, which with regard tothe repeat units comprise at least 80 mol % acrylic acid segments,wherein the dental material has a reduced tendency to gel.
 23. Thedental material of claim 13, wherein the polyacids are copolymers inwhich the proportion of the comonomers amounts to 0.1 to 20 mol %.
 24. Acontainer including at least one component of a dental material asclaimed in claim
 15. 25. A container including at least one component ofa dental material as claimed in claim
 17. 26. A container including atleast one component of a dental material as claimed in claim 19.